1. Field of the invention
The present invention relates to a thermal head recording device for use in a facsimile, a line printer or the like.
2. Description of the Prior Art
The thermal head recording device generally includes, as shown in FIG. 1, a thermal printer 1 having a plurality of heat generating elements 2 aligned horizontally in a line. The heat generating elements 2 are connected to a driver 3 which provides a binary-coded signal defined by a combination of numerals "0" and "1" to the heat generating elements 2. For example, if the driver 3 provides a binary-coded signal (111 . . . 111) with all digits being "1", all the heat generating elements 2 are actuated to effect recording by way of printing on a sheet to be recorded that moves pass the heat generating elements 2 in a direction indicated by an arrow. In this case, the sheet 4 is printed with a solid line comprising densely plotted dots. Therefore, by changing the combination of numerals in the binary-coded signal, it is possible to print not only the solid line, but also any broken line. After printing one line, the sheet 4 advances upwardly as indicated by an arrow in FIG. 1 for a predetermined pitch, ready for printing the next line. The driver 3 receives the binary-coded signal for one line alternately from memories A and B through a switch SW1 in a synchronized manner with the advance of the sheet 4, such that the memory A supplies binary-coded signal to the driver 3, e.g., for the odd lines and the memory B for the even lines.
The thermal head recording device further includes a receiver 5 which receives an image information in a coded form from an information transmission line 6, such as a telephone line in the case of facsimile. The receiver 5 decodes the coded image information into binary-coded signal and supplies the binary-coded signal of one line alternately to the memories A and B through a switch SW2. A signal transmission from the information transmission line 6 to the thermal printer 1 is further described below in connection with FIG. 2.
As shown in the first row of FIG. 2, the coded image information transmitted along the transmission line 6 includes image codes and EOL (end of line) codes occurring alternately. One image code includes one or more codes carrying information of one line. For a solid line, or a blank line, the image code includes a very simple and short code. Contrary, if one line is a complicated broken line, the image code includes many codes. Therefore, the length of one image code varies depending on the image to be printed. Upon receipt of one image code, the receiver 5 decodes the image code into binary-coded signal having a predetermined number of bits. For example, if the received image code is a simple code designating a solid line, the receiver 5 produces a binary-coded signal (111 . . . 111) with all digits being "1". Contrary, if the received image code is a simple code designating a blank line, the receiver 5 produces a binary-coded signal (000 . . . 000) with all digits being "0". Furthermore, if the received image code is a combination of many codes designating a complicated broken line, the receiver 5 produces a binary-coded signal having digits in combination of "0" and "1". The manner in which the image information is coded and decoded is based on "Recommendations" by CCITT (THE INTERNATIONAL TELEGRAPH AND TELEPHONE CONSULTATIVE COMMITTEE).
As understood from the foregoing, the length of each image code differs from other image codes, however, the length of each binary-coded signal is the same as other binary-coded signals.
The binary-coded signals as decoded by the receiver 5 are stored in the memories A and B and then are used for printing in the follow manner. For example, as shown in FIG. 2, a binary-coded signal corresponding to (N-1)th line is stored in the memory A. Then, upon receipt of the EOL code at a moment T1, the receiver 5 starts to receive image code of Nth line which is decoded to binary-coded signal and is stored in the memory B. Furthermore, immediately after the moment T1, the binary-coded signal of (N-1)th line stored in the memory A is fed to the driver 3 for effecting the printing in a predetermined sequence. The printing of (N-1)th line completes at a moment T2. The operation carried out in a time interval T1-T3 is understood as one cycle of operation in which decoding and memorizing for one line, e.g., Nth line, are carried out by the receiver 5 and the memory A or B, and at the same time, the printing of one line, e.g., (N-1)th line, is carried out by the driver 3 and the thermal head 1.
The next cycle of operation starts upon receipt of the next EOL code at a moment T3. At the moment T3, the receiver 5 starts to receive image code of (N+1)th line which is decoded to binary-coded signal and is stored in the memory A. Furthermore, immediately after the moment T3, the binary-coded signal of Nth line stored in the memory B is fed to the driver 3 for effecting the printing in a predetermined sequence. The printing of Nth line completes at a moment T4.
Thereafter, another cycle of operation is carried out upon receipt of the next EOL code at a moment T5. In this manner, a number of cycles of operations are carried out, thereby establishing a plane image as the horizontal line images align vertically.
In the above description, since the length of one image code differs from other image codes, the time length of one cycle of operation is not constant. In other words, a time interval T1-T3 is not equal to a time interval T3-T5. Whereas, since the length of one binary-coded signal is equal to the other binary-coded signals, the printing time T1--T2 for printing the (N-1)th line and the printing time T3-T4 for printing the Nth line are equal to each other.
From the above facts, it can be said that non-printing periods T2-T3 and T4-T5 vary in each cycle of operation.
During the non-printing periods T2-T3 and T4-T5, the heat generating elements 2 are maintained deactivated and, therefore, it can be said that during such non-printing periods the heat generating elements 2 are cooled down, resulting in cooling down of the surrounding atmosphere. Since the temperature of the surrounding atmosphere is in relation to the darkness, or contrast, of printing such that the print becomes dark as the surrounding temperature becomes high and, on the contrary, the print becomes light as the surrounding temperature becomes low, the temperature change of the surrounding atmosphere results in uneven darkness of the printed sheet. Such an uneveness is particularly apparent between the starting portion and the ending portion of the printed sheet. For example, under such circumstances that the surrounding temperature is 0.degree. C. and that pulse width of each pulse in the binary-coded signal is 1.2 ms with the length of one cycle of operation being 100 ms and that the printing period is 10 ms, the darkness of the printed portion showed 50% difference.
In order to improve the unevenness of the darkness of the printed sheet, approaches have been made and one of which is disclosed in Japanese Patent application laid open to public (Tokkaisho) No. 52-80846. According to the thermal head recording device disclosed in Tokkaisho No. 52-80846, an auxiliary heat source is provided in addition to the heat generating element that effects the printing so that the heat generating element as well as the surrounding atmosphere is maintained in a desired temperature. According to this arrangement, it is necessary to provide the auxiliary heat source and its associated parts such as a power source for the auxiliary heat source and a control circuit for controlling the auxiliary heat source, resulting in bulky in size and complicated in arrangement.
Another prior art thermal head recording device is disclosed in U.S. Pat. No. 3,577,137 in which a voltage to be supplied to the heat generating element is controlled to generate a predetermined temperature from the heat generating element. According to this arrangement, it is necessary to further provide a circuit for controlling the voltage level resulting in complication of the circuit, and furthermore, such an arrangement is not suitable for the control in digital form.